The invention relates to technology used for taking core samples from the seabed using a drill that is lowered and controlled remotely from a ship.
Conventionally taking core samples from the seabed has been achieved by either a technique known as piston coring or diamond coring.
Diamond coring is achieved by using conventional core barrels with diamond set bits. Commonly this technique is used when drilling rock.
On the other hand piston coring is particularly suited to seabed operations where typically the seabed is covered with a layer of sedimentary material that is too soft to core successfully using standard diamond coring system.
The current invention relates to improvements in this latter method and therefore the following description deals in detail with that type of prior system.
It is well known to take short samples with core sampling tubes such as the Shelby tube However, it has been found that the friction on the sample acting on the inner walls of the tube quickly builds up to prevent the entry of new material. This means that the tube becomes effectively a solid rod and displaces the sediment without any further winning of sample.
This effect is particularly damaging when there are layers of very soft and harder material, as the friction of the harder material prevents any, or at most little, of the soft material entering the tube. The sample in the tube then consists almost entirely of the harder material.
Other conventional sampling techniques for the seabed take advantage of the water pressure at depth to take longer and more representative samples by use of tethered piston coring technology. In such technology the drill frame located near the seabed by support means and includes a hydraulic feed cylinder and rope and pulley system. The feed cylinder causes the core sampling tube to be pushed into the seabed. A piston is installed inside the sampling tube and includes seals to prevent leakage past the piston. The piston is supported from the frame by tether rope, so that, as the tube is pushed into the seabed, the piston is constrained to remain stationary.
If the friction of the material in the tube creates enough force to overcome the hardness of the material entering the bottom of the tube, the material in the barrel will try to move down with the tube. Providing that the material is essentially impervious, this will create a reduced pressure under the tethered piston. The difference in pressure between that at the bottom of the tube and that under the piston is then available as an additional force to overcome the friction of the material in the tube.
The reduced pressure under the piston is self-regulating as it is generated by the friction in the tube and the pressure gradient down the tube is proportional to the friction in each part of the tube. This means that a complete sample of the seabed is obtained, complete with soft and hard layers.
It will be apparent that this process becomes more effective with increasing water depth because the available reduction in pressure increases. It is essentially ineffective on or near the surface.
Whilst this system is effective, it has been difficult to apply this method to a drill that has a segmented drill string made up of a variable number of drill rods depending on penetration depth, because there is no practical way of connecting the tether rope to the piston in the core barrel at the bottom of the drill string.
Accordingly further investigations have been carried out in attempt to improve the applicability of a piston based coring system.
It is an object of the present invention to overcome the limitations of current piston coring systems, more particularly, to obviate the need to use a structurally tethered piston.
Accordingly in one aspect of the invention, a method of acquiring a core sample of seabed material into a core sampling tube having an upper end, a lower open end and a substantially cylindrical chamber extending there between, comprising the steps of urging the core sampling tube into the seabed and simultaneously withdrawing fluid from the upper end of the core sampling tube at a rate sufficient to cause the seabed material to be drawn into the core tube at substantially the same rate as the core tube penetrates the seabed.
Preferably, the step of withdrawing fluid from the upper end of the core sample tube comprises withdrawing the fluid through a conduit means connected at one end to the core sampling tube and connected at its other end to a remote means for withdrawing fluid. Preferably, the steps of urging the core sample into the seabed and withdrawing fluid from above the seabed material is performed by a combination of remotely coordinated hydraulic fluid power means. Typically, the coordination of the hydraulic fluid power means comprises the steps of pumping hydraulic fluid into a first hydraulic means to urge the core sampling tube into the seabed and simultaneously pumping hydraulic fluid into a second hydraulic means to withdraw fluid from the upper end of the core sampling tube.
It will be understood that a freely movable piston may or may not be located in the core sampling tube. It will be included where there is a significant risk that seabed material may also be withdrawn from the sampling tube.
Accordingly, it is preferred to provide the core sampling tube further with a piston sealingly engaging and movable within the cylindrical chamber above the seabed material entering the core tube, and the step of withdrawing fluid is from above the piston such that the piston is maintained substantially stationary.
In a separate aspect of the invention which is adapted to be used with the method described above, a core sampling tube is provided comprising a core barrel having an upper end with a fluid inlet/outlet, an open lower end and a substantially cylindrical chamber extending there between to receive seabed material.
Preferably, the core sampling tube further comprises a piston sealingly engaging the cylindrical chamber and movable axially within the cylindrical chamber in response to fluid flow through the inlet/outlet.
Preferably, the core sampling tube further comprises an adaptation at the upper end to provide sealing means to permit a leak free connection to the conduit connectable between the core sampling tube and the remote means for withdrawing fluid.
In a further separate aspect of the invention which is adapted to be used with the method and core sampling tube described above, a seabed coring system is provided comprising:
(a) a core sampling tube described above;
(b) first hydraulic fluid power means to urge the core sampling tube into the seabed;
(c) second hydraulic fluid power means to withdraw fluid from the core sampling tube above the seabed material; and
(d) first conduit means connected between the core sampling tube and the second hydraulic fluid power means;
xe2x80x83wherein the first hydraulic fluid power means and the second hydraulic fluid power means are coordinated such that the seabed material will enter the core sampling tube at substantially the same rate as the core tube penetrates the seabed.
Preferably, the seabed coring system further comprises a piston,sealingly engaging and movable within the cylindrical chamber of the core sampling tube above the seabed material entering the core tube.
Preferably, the first hydraulic fluid power means comprises a substantially cylindrical chamber, a piston sealingly engaging the cylindrical chamber and movable axially within the cylindrical chamber to define a first chamber and a second chamber, and a piston rod connected to the piston and extending through and from the second chamber so that selective application of hydraulic pressure to the first chamber will urge the core sampling tube into the seabed.
Preferably, the second hydraulic fluid power means comprises:
(a) a first sub hydraulic means including a substantially cylindrical chamber, a piston sealingly engaging the cylindrical chamber and movable axially within the cylindrical chamber to define a third chamber and a fourth chamber, a piston rod connected to the piston at one end thereof and extending through the fourth chamber;
(b) a second sub hydraulic means comprising a substantially cylindrical chamber, a piston sealingly engaging the cylindrical chamber and movable axially within the cylindrical chamber to define a fifth chamber, the piston rod of the first sub hydraulic means having its other end connected to the piston; and
(c) second conduit means connected between the second chamber of the first hydraulic means and the fourth chamber of the first sub hydraulic means;
xe2x80x83wherein, as the core sampling tube is urged into the seabed by the first hydraulic fluid power means, hydraulic fluid is passed from the second chamber of the first hydraulic fluid power means into the fourth chamber of the first sub hydraulic means via the second conduit means to move the piston therein which in turn draws the piston of the second sub hydraulic means away from the first conduit means to cause the withdrawal of fluid from the core sampling tube.
Typically, the first conduit means consists in part of at least one hose with high collapse capability.
In another typical arrangement, the first conduit means consists in part of at least one drill rod with sealing means to provide a leak free joint between the drill rod and any preceding drill rod.
It will be appreciated that three separate aspects of the invention have been disclosed. namely, a method of acquiring a core sample from a seabed, a core sampling tube and a system (apparatus) for acquiring a core sample. Whilst the description explains preferred embodiments of each aspect in combination with one another, such aspects are not so interdependent and should not be so construed.